1. Field of the Invention
This invention relates to insulation materials and methods, for example, for internal and external insulating applications in rocket motors and, more particularly, to insulation having carbon fibrous components. The novel insulation of this invention is especially useful for insulating the interior surface of the casing of a solid propellant rocket motor, among other applications.
2. State of the Art
Although there are many variations to the configuration and construction of a rocket motor, rocket motors generally comprise an outer motor casing for housing an energetic fuel or propellant. In the case of a solid rocket motor, the outer motor casing houses a solid propellant formulated to undergo combustion while contributing to the production of adequate thrust for attaining propulsion of the rocket motor. Other types of rocket motors, such as hybrid motors, reverse-hybrid motors, and biliquid motors, make use of a liquid fuel and/or oxidizer. A rubber insulation layer and a liner protect the rocket motor casing from high temperature while the propellant burns.
Rocket motor casings are generally made of metal, a composite material, or a combination of metal and composite materials. Composite materials are typically prepared by winding a resin-impregnated fiber on a mandrel to shape the rocket motor casing. The resin may be any suitable type of curable resin, including, for example, an epoxy resin or a phenolic resin where the fibers are, for example, aramid fibers.
During operation, a heat-insulating layer or layers (insulation) protects the rocket motor casing from thermal effects and erosive effects of particle streams generated by combustion of the propellant. Typically, the insulation is bonded to the interior surface of the casing and is generally fabricated from a composition that, upon curing, is capable of enduring the high temperature gases and erosive particles produced while the propellant grain burns. A liner layer (liner) functions to bond the propellant grain to the insulating layer and to any noninsulated interior surface portions of the casing. Liners also typically have an ablative function, inhibiting burning of the insulation at liner-to-insulation interfaces. A number of liner compositions are generally known to those skilled in the art. An exemplary liner composition and process for applying the same is disclosed in U.S. Pat. No. 5,767,221.
The combustion of a solid rocket propellant generates extreme conditions within the rocket motor casing. For example, temperatures inside the rocket motor casing can reach 2,760° C. (5,000° F.). These conditions, along with the restrictive throat region provided along the nozzle passageway, combine to create a high degree of turbulence within the rocket motor casing. In addition, the gases produced during propellant combustion typically contain high-energy particles that, under a turbulent environment such as encountered in a rocket motor, can erode the rocket motor insulation. If the propellant penetrates through the insulation and liner, the casing may melt or otherwise be compromised, causing the rocket motor to fail. Thus, it is crucial that the insulation withstands the extreme conditions experienced during propellant combustion and protects the casing from the burning propellant.
In the past, candidates for making rocket motor insulation have included filled and unfilled plastics or polymers, such as phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, as well as ceramics, polyester resins, and the like. Plastics, however, tend to crack and/or blister in response to the rapid heat and pressure fluctuations experienced during rocket motor propellant combustion.
Rubbers or elastomers have also been used as rocket motor insulation materials in a large number of rocket motors. Cured ethylene-propylene-diene monomer (“EPDM”) terpolymer is a specifically advantageous rubber used alone or in blend and is often selected because of its favorable mechanical, thermal, and ablative properties. However, in high velocity environments, the ablative properties of elastomers are sometimes inadequate for rocket motor operation unless the elastomers are reinforced with suitable fillers. The criticality of avoiding high erosion rates is demonstrated by the severity and magnitude of risk of failure due to erosion. Most insulation is, of necessity, “man-rated” in the sense that a catastrophic failure can result in the loss of human life. The monetary cost of failure in satellite launches is well publicized. Additionally, the tensile strength and tear strength of unfilled elastomers may not be sufficiently high to withstand and endure the mechanical stresses that the elastomer is subjected to during processing.
It is known in the art to load elastomeric (e.g., cured EPDM) insulation materials with fillers, such as carbon fibers and/or silica, to improve the ablative and physical properties of the insulation.
Current silica-filled elastomeric insulation materials are electrically insulating, exhibiting high volume resistivities and, hence, a poor ability to dissipate static charge. The ability to dissipate static charge is considered to be an important quality for the thermal insulator. An insulator possessing this quality is able to bleed off or dissipate charges that build up on the insulator surface. An insulator having a high electrical resistivity does not dissipate static charge timely, thus creating a potential for static charge to build up to a dangerous level. When the electric field increases to the point that breakdown of the air occurs or a path to ground for the static charge is inadvertently provided, the discharge can be dangerous. Physical harm to personnel and flash fires are possible. Conventional silica-filled EPDM insulation is electrically insulating, having resistivities ranging from 1×1014 to 1×1016 Ohm·cm. An insulator is considered to be able to dissipate static charge if its volume resistivity is in the range of from 1×105 to 1×1012 Ohm·cm. Asbestos-filled NBR, which is one of the few currently used insulation materials that is considered to be able to dissipate static charge, has a volume resistivity in the range of 1×1011 to 1×1012 Ohm·cm. However, the use of asbestos in rocket motors has lost favor due to reported health dangers associated with asbestos.
Conventionally, carbon fiber filler used in elastomeric insulation is prepared from spun organic fiber, which is graphitized and cut to desirable dimensions. An example of a rocket motor insulation composed of EPDM (NORDEL® 1040) as the primary terpolymer is commonly known in the industry as STW4-2868 thermal insulation and has approximately the following composition:
IngredientFunctionParts by WeightNORDEL ® 1040Primary EPDM80Terpolymer baseNeoprene FBSecondary polymer base20Zinc oxideActivator5SulfurCurative1HAF carbon blackPigment1MBTAccelerator1AGERITE ® Resin DAntioxidant2Acerite HPSAntioxidant1TelluracAccelerator0.50SulfadsAccelerator0.75VCM carbon fibersFiller41Total parts by weight153.25
Although many organic-based fibers can be dispersed in the EPDM without too much difficulty, the homogeneous dispersion of carbon fibers in EPDM presents a difficult processing problem. Specifically, the mixing process is complicated by the fragility of the carbon fibers. Mixing carbon fibers into a solid elastomer under high shear physically deteriorates the carbon fibers into smaller particles or shreds, thereby negating the advantageous physical attributes that the carbon fibers would otherwise have contributed to the insulation.
Conventionally, the problem of carbon fiber fragility has been addressed by dissolving the polymer binder into solution with an appropriate organic solvent to lower the viscosity of the polymer. Suitable solvents include hydrocarbons such as hexane, heptane, and cyclohexane. The frangible graphitized carbon fibers can then be mixed with the solution in, for example, a sigma-blade mixer without significant breakage of or damage to the carbon fibers. The material is then sheeted out and the solvent is allowed to evaporate at ambient atmosphere or in an oven.
The use of solvent in this processing technique presents several drawbacks. For example, solvent-processing techniques, such as those conventionally used to disperse carbon fibers in EPDM terpolymer, are relatively expensive. Material costs are increased by the use of solvents, as are processing costs, since additional workers and special equipment are required to handle and process the solvents. Further, considerable costs and worker safety issues are associated with the disposal of hazardous volatile organic solvents.